Bactericidal/permeability-increasing protein (hereinafter "BPI") is a cationic protein which binds to the lipid A portion of bacterial lipopolysaccharide (hereinafter "LPS"). Binding of the BPI protein to bacterial LPS increases the envelope permeability of susceptible gram negative bacteria. Ooi, et al., J. Biol. Chem., 262:14891 (1987). BPI also binds to soluble LPS. Human BPI protein has been isolated from polymorphonuclear neutrophils (hereinafter "PMNs") by acid extraction combined with either ion exchange chromatography or E. coli affinity chromatography. Elsbach, et al. J. Biol. Chem., 254:11000 (1979); Weiss et al., Blood, 69:652 (1987).
The holo-BPI protein isolated from human PMNs has potent bactericidal activity against a broad spectrum of gram-negative bacteria. Elsbach, et al., J. Biol. Chem., 254:11000 (1979). This antibacterial activity appears to be associated with the amino terminal region of the isolated human holo-BPI protein. In contrast, the C-terminal region of the isolated human BPI protein displays only slightly detectable anti-bacterial activity. Ooi, et al., J. Exp. Med., 174:649 (1991). Human DNA encoding BPI has been cloned and the amino acid sequence of the encoded protein has been elucidated. Gray et al., J. Biol. Chem., 264:9505-9509 (1989); U.S. Pat. No. 5,198,541.
Immunoglobulins comprise a family of proteins with numerous structural similarities, but also important structural differences which result in differences in antigen binding properties and other biological activities. For example, the IgG isotype antibodies have the longest serum half life and are susceptible to placental transfer. The most potent anti-viral activity is associated with the IgA isotype antibodies; while the IgM isotype antibodies possess the greatest antibacterial efficacy. Stites, et al. Basic and Clinical Immunology, p. 32 (Appleton & Lange, 6th ed. 1987). Within each isotype of antibody in the immunoglobulin family there exist several subclasses and allotypic variations. Id.
Members of the so-called "immunoglobulingene superfamily" generally possess extracellular domains which are characterized by multiple looping due to the formation of disulfide bridges. Such looping occurs in the heavy chain constant domains, (designated CH1, CH2, CH3, and CH4) of immunoglobulin molecules. Non-immunoglobulin compounds possessing multiple looping domains which are homologous to those present in members of the immunoglobulin superfamily also exist and certain of these have been termed "adhesons". See, e.g., PCT Application No. WO89102922, published Apr. 6, 1989; Capon et al., Nature, 337:525-531 (1989).
Of particular interest to the present invention are reports of the recombinant synthesis of hybrid fusion proteins involving a portion of an adheson as the first component and an immunoglobulin heavy chain constant region as a second component of the fusion. See, e.g. Harris, Eur. J. Biol. Chem. 194:611-620 (1990) and Capon et al., supra, addressing the formation of CD4/IgG fusions. The structural design rationale for such a molecule is based on the observation that the adheson component of the fusion has a structure similar to that of the immunoglobulin component, and would therefore be expected to fold in a manner which is complementary to that of the immunoglobulin component. See Gascoigne, et al., P.N.A.S. (USA), 84:2936-2940 (1987) addressing recombinant chimeric T-cell receptor-immunoglobulin proteins. See also Mariuzza, et al., J. Biol. Chem., 264(13):7310-7316 (1989); Goverman, et al., Cell 60:929-939 (1990); Gregoine, et al. P.N.A.S. (USA), 88:8077-8081 (1991); Bismuth, et al., Molecular Immunol., 27(11):1127-1136 (1990) (addressing similar T-cell receptor-immunoglobulin fusions). A soluble CD44-immunoglobulin fusion protein has also been reported. Aruffo, et al., Cell 61:1303-1313 (1990).
Ashkenazi, et al., P.N.A.S.(USA), 88:1035 (1991) report protection against endotoxic shock through use of a chimeric immuno-adhesin protein (an adheson variant) which acts as a tumor necrosis factor (TNF) antagonist. The TNF antagonist reported therein is a hybrid fusion protein wherein the extracellular portion of a TNF receptor (TNFR) protein is fused to a constant domain of a human IgG heavy chain. This TNFR-IgG fusion reportedly binds to and blocks the cytotoxic effect of TNF on actinomycin-D treated cells and provides protection against endotoxin challenge when administered prior to the endotoxin.
Significantly, all of the fusion proteins described above have involved molecules that are expressed on the surface of cells as integral membrane proteins and, with the exception of CD44 and the TNF receptor, have possessed structures characteristic of the immunoglobulin gene superfamily.
Published PCT application WO92/03535 reports the construction of fusion of an amino-terminal portion of BPI to cDNA encoding the constant domain of IgG. However, that report fails to indicate how such protein-cDNA constructions are made and further fails to teach how many other types of BPI-Ig fusion may be constructed.